Many methods have been proposed for the generation of gradient images from discrete dots of ink.
Combinations of different density solvent based inks in an ink jet printing method have been shown to be a suitable approach to the generation of high gradient images from a discrete number of inks. U.S. Pat. Nos. 4,727,436; 4,860,026; 5,142,374; 4,713,746; and 4,713,701 all teach variations on methods and apparatus for combining inks. Suitable gradients are available using these and other techniques. Even with suitable gradients the image quality is still unsuitable due to image dot spreading which occurs as a result of the carrier solvent, such as water or an organic, and the ink diffusing into the media. Another major disadvantage of solvent based ink jet systems is the solvent which must be absorbed by the media or evaporated after printing. Evaporation of the solvent is environmentally unsatisfactory particularly when non-aqueous solvents are employed. It has been a long standing goal of skilled artisans to decrease the amount of ink used to form an image which, in-turn, decreases the image dot spread and lowers cost.
Phase change ink printing provides some advantages over solvent based ink jet systems. Specifically, there is no solvent since the phase change ink is a solid at room temperature and a liquid at coating temperatures. One disadvantage of phase change ink printing is the inability to easily vary drop size on demand. Discrete drop sizes limit the gradient levels available with conventional phase change ink printing methods due to the lack of continuously variable ink density levels. Phase change ink printing does allow for the placement of multiple dots at a given position which increases the contrast available to some extent. When multiple dots are applied image resolution and image durability deteriorate due to the appearance of ink islands occurring as a result of the stacking of solid ink. Phase change inks and printing techniques are described, for instance, in U.S. Pat. Nos. 5,372,852 and 5,276,468 and in European Patent Applications 0 566 259 and 0 604 025.
It would be highly advantageous to combine the dry printing capabilities of phase change ink printing with the ink combining methods of solvent based ink jet printing to achieve a superior print with high resolution and contrast. Efforts towards this goal have been thwarted and the method has been considered to be abandoned by skilled artisans due to the loss of resolution and poor image durability resulting from the ink islands.
The present invention provides a method for eliminating the problems associated with combining phase change inks of different densities. The resulting image exhibits excellent gradation without discontinuities and provides a superior method of printing.